Phthalimidoperoxyalkanoic acids, for example 6-phthalimidoperoxyhexanoic acid (PAP), are highly efficient bleaching agents. Their use in both solid and liquid washing or cleaning agents has been repeatedly proposed.
When phthalimidoperoxyalkanoic acid-containing washing-agent formulations are used in commercially usual washing machines, however, traces of corrosion can be found after approximately at least 50 washing cycles, these traces being found often on the mounts of the heating elements and on the heating elements themselves, largely regardless of whether nickel-plated material is involved. With nickel-plated heating rods in particular, it may happen that firstly the Ni layer of the nickel-plated heating elements is removed directly at the contact point with the mount made of chromium steel or stainless steel, for example Nirosta® 4301. A brownish “halo”, which presumably is rust, can form around this point. Rust likewise often occurs at the heating-element mount in the immediate vicinity of the contact point. The heating element itself can be covered with small brownish discolorations that, however, are normally not anywhere near as pronounced at other sites as they are at the contact point with the mount. When such corrosion phenomena occur, premature failure of the heating element must be expected. The same corrosion effect also occurs analogously in automatic dishwashers.
In EDX measurements on steel pieces that have been treated with phthalimidoperoxyalkanoic acid-free and, for comparison, with phthalimidoperoxyalkanoic acid-containing washing liquor, nitrogen can be detected on the metal surface after treatment with a phthalimidoperoxyalkanoic acid-containing bath. The conclusion resulting from this is that phthalimidoperoxyalkanoic acids have an affinity for the metal surface, and adsorb onto it.
The corrosion potential between nickel and steel in a phthalimidoperoxyalkanoic acid-containing washing bath is time-dependent. This change in potential over time can be attributed to breakdown of the phthalimidoperoxyalkanoic acid.
With no intention to be confined to this theory, this adsorption of phthalimidoperoxyalkanoic acid onto the metal surface is possibly the cause of the corrosive effect, since in this context, the phthalimidoperoxyalkanoic acid (constituting an oxidizing agent) is present at a high concentration directly at the surface of the cathode.
It has been found, surprisingly, that this problem can in no way be solved by the use of any known corrosion inhibitors.